Endless track-driven vehicles are commonly used off-road in difficult terrain and under difficult terrain conditions, such as in mud, snow, sand, and tundra. For example, tracked vehicles are used in snow country for grooming ski slopes and snow mobile trails, for transporting skiers to back-country slopes, for ski resort maintenance work, and for snow and mountain rescue. They are also used in various types of terrain for utility company maintenance work, and for oil exploration and oil pipeline maintenance in arctic tundra.
Tracked vehicles are generally of two types. Many are two-tracked in which a pair of endless drive track units, one on each of the opposite sides of the vehicle, support and drive the vehicle. Others are four-tracked, in which four separately driven and independently suspended drive track units, two in front and two in the rear, support and drive the vehicle.
Four-tracked vehicles have certain advantages over two-tracked vehicles under extreme conditions such as on steep slopes and in very rough terrain because of the flexible independent suspensions of the track drive units and the constant power available to all of the track drive units, even while turning. Unlike a two-tracked vehicle which relies on the differential speed of the two tracks for turning, a four-tracked vehicle steers much like a wheeled vehicle. Its endless drive track units can be physically turned for steering.
In many tracked vehicles, such as the four tracked vehicles shown in U.S. Pat. No. 6,007,166, the lower, ground-engaging run of the track has been supported by a plurality of single-element guide wheels disposed substantially inline longitudinally of the track and generally engaging only a centralized region of the track. Although this system, with a single line or row of guide wheels, functioned adequately, it was found that substantial deflection of the track on opposite sides of the guide wheels was occurring. This deflection was caused generally by the high-point loading of the track by the guide wheels at the center of the lateral dimensions of the track. This could lead to premature failure of elements in the track due to high cyclical stresses. This same high-point loading of the track and its consequential deflection could also lead to premature failure of the track and reduce its effective traction. The high-point loading of the track was also transmitted through the track to the underlying terrain. In the case of sensitive terrain such as tundra, such loading could cause excessive damage to the terrain, especially with the endless track usually including traction bars or cleats on its outer surface for enhancing the vehicle's traction.
The drive track unit disclosed in U.S. Pat. No. 6,129,426, addressed the foregoing problems by providing guide wheel assemblies along the lower track run, with each assembly including multiple guide wheels mounted on a common guide wheel hub. This increased the guide wheel surface contact area across the width of the track, thereby reducing point loading of the track and consequential track deflection and wear, as well as terrain damage. Although this was a substantial improvement over the prior art and alleviated the aforementioned problems to some extent, it did not do so altogether. Consequently, some of the track wear and terrain disturbance problems still occurred, especially with all-terrain vehicles that carry heavy loads. Despite there being three separate wheel rims mounted on a common hub, there still remained essentially only a single wheel unit in each such guide wheel assembly, and the surface contact areas of the three rims with the track were still relatively small compared to the overall width of the track. Therefore, each such single three-wheel guide wheel assembly still left extensive outwardly and inwardly extending track surface areas across the width of the track unsupported on the lower track run. Accordingly, substantial track deflection and wear could still occur, as could substantial damage to sensitive terrain underlying the track area.
Another problem with the guide wheel assemblies of the track units of U.S. Pat. No. 6,129,426, was that such guide wheel assemblies were not provided throughout the entire lower track run of the track unit. It was still necessary to include at least one single guide wheel along the lower track run, to provide clearance for the drive wheel assembly of the track unit. Thus, single point loading of the track still occurred along at least a portion of the lower track run.
Furthermore, the drive track units of some prior all-terrain vehicles, especially four-tracked vehicles intended for use on sensitive terrain, have had endless tracks comprising one-piece, endless, molded rubber belts, as disclosed in both of the aforementioned U.S. Pat. Nos. 6,129,426 and 6,007,166, each with rows of inwardly extending hard rubber drive lugs positioned on the inner circumference of the belt. These lugs engaged and meshed with drive cogs on a drive sprocket wheel assembly of the drive track unit. Heretofore, such drive sprocket wheel assemblies have been expensive to fabricate and assemble because of their multiple separate parts. Such parts have included separate sprocket wheels bolted to a common sprocket drum, which in turn was bolted to an axle hub. The hub in turn, was bolted to an end flange of the drive axle itself. The resulting drive wheel assembly was also quite heavy and, especially when used in multiples in a four-tracked vehicle, added substantial weight to the vehicle, thereby increasing the loading of its often already heavily loaded drive tracks.
In the past, drive track units with rubber drive belts have also included outer, ground-engaging surfaces with molded hard rubber traction bars or cleats to improve traction. However, such cleats tend to exacerbate the ground disturbance problems arising from point- or concentrated loading of the belts by the guide wheels of such units.
Another problem that can arise with drive track units that support heavy loads and utilize rubber drive belts, is that the belts may stretch or “give” in use especially when high driving forces are applied by a sprocket wheel to the drive lugs of the belt. Such stretch can give use to so-called track bounce creating heavy vehicle vibrations, or even track jump, where the belt lugs jump from engagement with the sprocket drive cogs, causing the vehicle to lose power.